1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device having a circuit constituted by a thin film transistor (herein after, referred to as TFT). For example, the invention relates to an electro-optic apparatus represented by a liquid crystal display apparatus and a constitution of an electric apparatus mounted with an electro-optic apparatus as a part thereof. Further, in the specification, a semiconductor device generally indicates an apparatus capable of functioning by utilizing semiconductor properties and an electro-optic apparatus and an electric apparatus, mentioned above, pertain to the category.
2. Description of the Related Art
In recent years, there have been widely carried out researches on a technology in which an amorphous semiconductor film formed above an insulating substrate of glass or the like is subjected to laser annealing to thereby crystallize the amorphous semiconductor and promoting crystalline performance thereof. Silicon is frequently used for the amorphous semiconductor film.
In comparison with a quartz substrate which has frequently be used conventionally, a glass substrate is provided with advantages that the glass substrate is inexpensive and rich in workability and the glass substrate having a large area can easily be fabricated. This is the reason that the above-described researches are carried out. Further, laser is preferably used in crystallizing the amorphous semiconductor since the melting point of the glass substrate is low. Laser can provide high energy only to an amorphous semiconductor film without considerably rising temperature of a substrate.
Since a crystalline semiconductor is constituted by a number of crystal grains, a film thereof is referred also as a polycrystal semiconductor film. A crystalline semiconductor film formed by being subjected to laser annealing is provided with high mobility and therefore, a thin film transistor (TFT) is formed by using the crystalline semiconductor film and the crystalline semiconductor film is intensively utilized in, for example, a liquid crystal electro-optic apparatus of a monolithic type in which TFTs for driving a pixel and for a drive circuit are fabricated above one sheet of a glass substrate.
Further, there is preferably used a method of carrying out laser annealing by shaping pulse laser beam such as excimer laser to constitute a square spot of several centimeters square or a linear shape having a length equal to or larger than 10 cm at an irradiated face or a vicinity thereof and scanning the laser beam (or moving an irradiated position of laser beam relatively to an irradiated face) since the method is provided with high productivity and is excellent industrially.
Particularly, when linear beam is used, the productivity is high since different from a case of using laser beam in a shape of a spot where scanning in front and rear direction and left and right direction is needed, laser can be irradiated to a total of an irradiated face by scanning the linear beam only in a direction orthogonal to a longitudinal direction thereof. Laser is scanned in the direction orthogonal to the longitudinal direction since the direction is the most efficient scanning direction. Owing to the high productivity, currently, the main stream is being established by using linear beam produced by shaping pulse oscillated excimer laser beam by a pertinent optical system.
FIG. 1 shows an example of a constitution of an optical system for shaping the shape of laser beam in a linear shape at an irradiated face or in the vicinity thereof. The constitution is extremely general and the constitution of FIG. 1 is applied to all of the optical systems. The constitution not only converts the shape of laser beam into a linear shape but, simultaneously, homogenizes energy of the laser beam at an irradiated face. Generally, an optical system for homogenizing energy of beam is referred to as a beam homogenizer.
When excimer laser as an ultraviolet ray is used for a light source, all of the mother material of the optical system is preferably constituted by quartz. This is because high transmittance is achieved. Further, there is preferably used a coating having 99% or more of transmittance with respect to a wavelength of excimer laser used.
First, an explanation will be given of a side view of FIG. 1. Laser beam emitted from a laser oscillator 61 is split in a direction orthogonal to a direction of progress of the laser beam by cylindrical array lenses 62a and 62b. In the specification, the above-described direction is referred to as vertical direction. When a mirror is put to a middle of an optical system, the vertical direction is bent in a direction of light bent by the mirror. According to the constitution, four splits are constituted. The splits of the laser beam are temporarily summarized into a single piece of laser beam by a cylindrical array lens 64. The laser beam is reflected by a mirror 67 and thereafter focused again into a single piece of laser beam at an irradiated face 69 by a doublet cylindrical lens 68. A doublet cylindrical lens designates a lens constituted by two sheets of cylindrical lenses. Thereby, there are determined homogenizing of energy of linear beam in the width direction and a length thereof in the width direction.
Next, an explanation will be given of a top view of FIG. 1. Laser beam emitted from the laser oscillator 61 is split in a direction orthogonal to the direction of progress of the laser beam and a direction orthogonal to the vertical direction by a cylindrical array lens 63. In the specification, the direction is referred to as horizontal direction. When a mirror is put to a middle of an optical system, the horizontal direction is bent in a direction of light bent by the mirror. According to the constitution, seven splits are constituted. Thereafter, the laser beam is synthesized into a single piece thereof by the cylindrical lens 64. Thereby, there are determined homogenizing of energy in the longitudinal direction of the linear beam and a length thereof.
The above-described lenses are made of synthesized quartz to correspond to excimer laser. Further, surfaces thereof are provided with coatings to transmit excimer laser excellently. Thereby, transmittance of excimer laser per lens becomes equal to or larger than 99%.
By irradiating the linear beam shaped by the above-described constitution in an overlapping manner while gradually shifting the laser beam in the width direction, an amorphous semiconductor can be crystallized or crystalline performance thereof can be promoted by subjecting an entire face of the amorphous semiconductor to laser annealing.
Next, a description will be given of a typical method of fabricating a semiconductor film constituting an object of irradiation. First, as a substrate, there is prepared a Corning 1737 substrate of 5-inch square having a thickness of 0.7 mm. An SiO2 film (silicon oxide film) having a thickness of 200 nm is formed above the substrate by using a plasma CVD apparatus and an amorphous silicon film (hereafter, expressed as a-Si film) having a thickness of 50 nm on the surface of the SiO2 film. The substrate is exposed to an atmosphere of nitrogen gas at temperature of 500 degree for 1 hour to thereby reduce a hydrogen concentration in the film. Thereby, laser resistance of the film is remarkably promoted.
As a laser apparatus, there is used XeCl excimer laser (wavelength 308 nm, pulse width 30 ns) L3308 made by Lamda Co. Ltd. The laser apparatus is provided with a function of emitting pulse-oscillated laser and outputting energy of 500 mJ/pulse. The size of the laser beam is 10 mm×30 mm (both are half value widths at a beam profile) at an outlet of the laser beam. In the specification, the outlet of laser beam is a plane orthogonal to the direction of progress of the laser beam immediately after emitting the laser beam from a laser irradiating apparatus.
A shape of laser beam emitted by excimer laser is generally rectangular and falls in a range of about 3 through 5 when expressed by an aspect ratio. The intensity of laser beam shows a Gaussian distribution such that the more proximate to the center of the laser beam, the stronger the distribution. The size of the laser beam is converted into linear beam of 125 mm ×0.4 mm having a uniform energy distribution by an optical system having the constitution shown by FIG. 1.
FIGS. 2A, 2B, 2C and 2D show a behavior of a state of irradiating linear beam by 2 pulses viewed from an upper face thereof and according to FIGS. 2A through 2D, a pitch of overlapping the beam width (half value width in beam profile) of the linear beam is changed. When laser is irradiated to the above-described semiconductor film, it is found that the pitch of overlapping the beam width is most pertinent to be around 1/10 of the beam width of the linear beam as shown by FIG. 2A. Thereby, the uniformity of crystal quality in the film of the semiconductor film is promoted. According to the above-described example, the half value width is 0.4 mm and accordingly, laser beam is irradiated by setting a pulse frequency of excimer laser to 30 Hz and scanning speed to 1.0 mm/s. At this occasion, an energy density at the irradiated face of laser beam is set to 420 mJ/cm2. The above-described method is an extremely general method used for crystallizing a semiconductor film by using linear beam.
In carrying out laser annealing, laser beam is shaped into a linear beam having a linear shape at an irradiated face or in the vicinity thereof by using an optical system as shown by FIG. 1. As shown by FIG. 2A, the pitch of overlapping of the linear beam in the beam width is set to around 1/10 of the beam width.
Further, as shown by FIG. 3, the wavelength of excimer laser is 308 nm and accordingly, the absorption coefficient at the wavelength is 1.38×106 cm−1 for an amorphous silicon film and 1.56×106 cm−1 for a polycrystal silicon film and accordingly, the absorption coefficients of the amorphous silicon film and the polycrystal silicon film are substantially the same.
From the above-described, when laser annealing is carried out by excimer laser, recrystallization is carried out by a number of times with respect to a region which has been crystallized once. Therefore, a dispersion in a grain size is caused.
Further, in the current state, the longitudinal length of the linear beam is about 100 mm. Even when the longitudinal length of the linear beam is expanded by using a beam expander, a limit of the length is about 150 mm in consideration of uniformity and energy density of the linear beam.
Meanwhile, large area formation of a substrate used has been progressed and as a large area substrate, there has been used, for example, a substrate of 600 mm×720 mm, a substrate of 320 mm×400 mm or a substrate of 8 inches (diameter: about 200 mm) in a circular shape. FIG. 4 shows an example of a method of irradiating the linear beam to such a large area substrate.
FIGS. 4A, 4B and 4C and FIGS. 27A, 27B and 27C show an example of irradiating the linear beam having the longitudinal length of 150 mm to a substrate of 320 mm×400 mm formed with an amorphous semiconductor film by scanning the linear beam (or moving a position of irradiating the linear beam relative to an irradiated face). According to a method of irradiating the linear beam as shown by FIGS. 4A, 4B and 4C and FIGS. 27A, 27B and 27C, regions of scanning the linear beam overlap at a central portion of the substrate (FIG. 4A, FIG. 27A) or the central portion of the substrate is not irradiated (FIG. 4B, FIG. 27B). Further, there is pointed out a method of irradiating the linear beam such that ends of irradiation by the linear beam are brought into contact with each other at the central portion of the substrate as shown by FIG. 4C and FIG. 27C.
When the linear beam is irradiated as shown by FIG. 4A and FIG. 27A, as has already been described, the absorption coefficients of excimer laser with regard to an amorphous silicon film and a polycrystal silicon film are almost the same and accordingly, at the central portion of the substrate which is an overlapped portion of the regions of scanning the linear beam, recrystallization is carried out by a number of times and a dispersion in grain size is caused. Therefore, even when TFT is fabricated by using the central portion of the substrate and electric properties thereof are measured, excellent properties are not achieved.
When the linear beam is irradiated as shown by FIG. 4B and FIG. 27B, laser annealing is not carried out at the central portion of the substrate and accordingly, the amorphous silicon film is not crystallized and crystalline performance of the amorphous silicon film differs from that of the crystalline silicon film provided by being subjected to laser annealing. Even when TFT is fabricated by using such a silicon film and electric properties are measured, the properties at the central portion of the substrate are significantly deteriorated.
When the linear beam is irradiated as shown by FIG. 4C and FIG. 27C, the energy density at both ends of the linear beam in the longitudinal direction is considerably lower than that at a vicinity of the center of the linear beam and therefore, the crystalline performance is deteriorated at the central portion of the substrate where ends of irradiation of the linear beam are present. Even when TFT is fabricated by using such a silicon film and electric properties are measured, a dispersion in the electric properties is caused in the substrate.
That is, as shown by FIGS. 4A through 4C and FIGS. 27A through 27C, when laser annealing is carried out by scanning the linear beam to a large area substrate, in any of the cases, there is produced a region having poor crystalline performance and even when TFT is fabricated with the region as an activation layer, excellent properties cannot be achieved in electric properties of the TFT.